1. Field of the Invention
This invention relates to a device for adjusting the clearance between a brake shoe and a brake drum.
2. Description of the Prior Art
Hitherto, it has been a common practice to adjust the clearance of the brake shoes and the brake drum of a motor vehicle by utilizing an adjusting lever pivoted in the brake drum for rotating a star wheel to extend a member, such as a strut of adjustable length, acting on the brake shoes. Many different methods have been proposed for operating the adjusting lever. In one of these methods, the adjusting lever is operated by means of the parking brake. In this system, a parking lever, actuated by the parking brake, pivots the adjusting lever against a bias. When the parking brake is released, the star wheel is rotated, with the resulting extension of the length of the strut. Thus, the clearance between the shoe and the drum may be maintained at an optimum value.
However, a device which adjusts the clearance of the brake shoes and brake drum by the operation of the parking brake develops a difficulty when the parking brake is operated at the same time as the main brake. In this system the brake shoes are biased against the ends of the strut by a spring, which bias acts on the sides of the star wheel to provide a normal rotational resistance. However, when the main brake is operated, the brake shoes are forced outwardly and the bias forces on the side of the star wheel are removed, allowing the star wheel to rotate with ease. On the other hand, the adjusting lever remains in engagement with the star wheel, so that the star wheel tends to rotate in either direction with the pivotal movement of the adjusting lever. In other words, the star wheel is rotated both when the parking brake is actuated and merely returns the star wheel approximately to its initial position due to the frictional engagement with the adjusting lever and the lack of substantial rotational resistance. As a result, the strut will not be extended, thus failing to effect any adjustment of the clearance between the shoe and the brake drum.
It often happens that both the foot brake and the parking brake are operated at the same time, so that the aforesaid clearance adjustment is not effected. More specifically, the parking brake is generally used when a motor has to be stopped on an upward slope, wait for a traffic signal or in parking. In such situations, a driver generally treads on the foot brake, and stops the vehicle, and then operates the parking brake, with the foot brake still in engagement.
Accordingly, the clearance between the brake shoes and the brake drum will not be adjusted. As lining wear increases with the number of braking operations, i.e. with the increase in the mileage of the vehicle, the braking ability of the vehicle is decreased. If such a case is left alone, eventual entire loss of braking may occur.